Quasi-static And Impact Mechanical Behavior Of AZ31 Magnesium Alloy At Low Temperatures

In this paper,the quasi-static tensile,quasi-static compression and impact behaviors of an annealed AZ31 magnesium alloy within the temperature range from room temperature to liquid nitrogen,have been investigated respectively by using material universal testing machine and Dynatup 9200 drop hammer impact test system,and the effect of low temperature on AZ31 magnesium alloy of corresponding properties were studied.The deformation-fracture features and relative mechanisms of AZ31 magnesium alloy under the above loading conditions at low temperatures were examined by applying optical microscope and scanning electron microscope(SEM)and EBSD analysis techniques.The results show that the yield strength and the ultimate tensile strength of the annealed AZ31 magnesium alloy increase in an approximately linear fashion,and the elongation continues to decrease as the stretching temperature continues to decrease,with the strain rate at 10-1s-1.The plastic deformation of the annealed AZ31 magnesium alloy under the quasi-static tensile condition is carried out with the base plane slip as the main deformation mechanism.While under lower temperature,twins are still activated in possible.But a few brittle characteristics are exhibited because of grain orientation under an unfavorable direction and the degree of twist deformation in a low level.When the temperature is lower than-100?C,a sign of brittle fracture begin to appear.The number of tearing ridges and dimples could be observed on fracture surfaces at room temperature.Moreover,the tearing ridges were entangled with each other.With the decrease of temperature,the number of tearing ridges decreased gradually,and the fracture features show brittle fracture characteristics at the same time.With the decrease of the compression temperature,the ultimate compression strength of the AZ31 magnesium alloy increases in an approximately linear way,and the yield strength is improved to a small extent when the strain rate at 10-1s-1.Compared with room temperature,the yield strength increased 31%,ultimate compression strength increased 57%,and the compressibility decreased 62%correspondingly at-196 ? C.Under the condition of low temperature quasi-static compression,prismatic plane slip as the main deformation mechanism influenced the process of plastic deformation,and the base plane slip has been restricted.At the lower temperature,with the coordination effect of twins,part of the grains' orientation were deflected moving to the pyramidal direction.However,harshly work by the pyramidal plane slip system,the promotion of the plastic deformationwere limited.After room temperature and low temperature compression,the macroscopic characteristics of ductile fracture of AZ31 magnesium alloy were not significant.With the decrease of temperature,the brittle fracture tendency heighten,and the brittle characteristic has been shown in the magnesium alloy more obviously at-196?C.It is shown that the impact strength of AZ31 magnesium alloy decreases with decreasing impact temperature,from 9.55 J at room temperature to 5.27 J at-190?C,and the ratio of crack propagation and crack initiation increased first and then decreased,reaching the maximum at-50?C.The results of EBSD analysis suggested that the main deformation mechanism for low temperature impact is the base plane slip.With the temperature dropping,the number of twins increased firstly and then decreased,reaching the maximum at-50 ?C,which indicated that the deformation twinning activated firstly and then restrained.With the decrease of impact temperature,fracture appearance of AZ31 magnesium alloy have little changes,and both room temperature and low temperature have been distributed with a large number of tearing edges and sizes of dimples,manifesting as quasi-cleavage fracture,which is due to a certain ability of plastic deformation have been possessed by AZ31 magnesium alloy at low temperatures.